1. Field of the Invention
The invention concerns a protective device for electrostatic sprayer equipment comprising, in a working area likely to contain inflammable mixtures, at least a terminal part of a high-voltage generator with an electrode for electrostatically charging a jet of sprayed material, at least one low-voltage electrical line connecting the terminal part of the high-voltage generator to a remotely sited electrical power supply equipped with cut-out means.
2. Description of the prior art
The use of high-voltage DC generators for supplying electrostatic sprayer equipment is well known as is the risk of ignition of the sprayed products or the vapors thereof as a result of the accidental production of electrical arcs, for example between the high-voltage electrode of the sprayer apparatus and the object to be sprayed, which is generally grounded. The risk is proportional to the quantity of electrostatic energy stored in the capacitance consisting of the high-voltage electrode, the output terminal of the generator and the cable connecting these. When the generator is sited far from the sprayer apparatus, at a distance of around ten meters, for example, the greater part of this capacitance consists of the connecting cable. The risk of arcing due to the capacitance of the cable is often reduced by inserting a damper resistance between it and the high-voltage electrode of the sprayer apparatus; if the capacitance of said electrode is low this protection is sufficient. If not, or in order to take account also of the risk of deterioration of the insulation of the high-voltage connecting cable, there is placed in the high-voltage circuit, generally between the ground terminal of the generator and the ground connection, a detector responsive to a current surge or to a rapid increase in the current and which, when it detects an anomaly in the high-voltage current, shuts down the high-voltage generator and connects its high-voltage terminal to ground in order to discharge rapidly to ground the capacitance which is charged to the high voltage. Another proposed solution is to reduce the capacitance charged to the high voltage by eliminating the connecting cable and juxtaposing the generator and the sprayer apparatus. This solution eliminates the risk associated with the capacitance of the high-voltage cable but not that associated with the capacitance of the high-voltage electrode of the sprayer apparatus. It has consequently been proposed, in particular in U.S. Pat. No. 4,187,527, to proceed as previously and to provide between the ground terminal of the high-voltage generator and the ground connection, in the high-voltage circuit, a detector responsive to an increase in the current by shutting down the generator and grounding the high-voltage electrode. As the generator may be grounded far away from the sprayer apparatus, in an area containing no combustible mixture, the high-voltage current increase detector is located in such an area and thus does not present any risk.
Other risks result from this solution, however. Even though the low-voltage input of the high-voltage generator and the device for grounding the high-voltage electrode are designed and constructed in such a way as to eliminate the risk of electrical arcing, it is difficult to protect effectively the low-voltage cables connecting them to their respective power supplies against this risk without making them stiff and heavy. This is unacceptable when the sprayer apparatus is mobile and light in weight, if it is mounted on a manipulator possessing several degrees of freedom, for example. Nor is it possible to limit the energy available in these low-voltage cables, for example by making them intrinsically safe using Zener barriers, since the power to be transmitted is too high. As this power is generally transmitted as a current of approximately one ampere with a voltage of several tens of volts, the risk of short-circuiting between the conductors of the low-voltage cables carrying these currents is easily eliminated by using maximum current circuit-breakers and/or fast-blow fuses. This does not apply to the risk of intermittent interruption of the circuits constituted by these cables, for example as a result of the rupture of a conductor due to fatigue resulting from the alternate bending imposed on it when manipulated. These intermittent interruptions do not result in current surges which can be detected by the aforementioned devices but they can nevertheless produce electrical arcs able to ignite inflammable mixtures as a result of the self-inductance and/or capacitance of the circuits in question.